A circuit arrangement having a substantially corresponding function principle is for example known from German Auslegeschrift No. 2 050 994. It operates such that at the testing point, a transmitter is fed with high-frequency oscillations, whereby this transmitter is converted to a damped condition on a first transfer coil by an arriving d.c. signal through a circuit element on one side of the testing point, which damped condition is evaluated through a second transfer coil for characterization of the d.c. signal on the other side of the testing point. A third transfer coil serves to feed the transmitter from a high-frequency generator. The d.c. signal is characterized by controlling a transistor with the half-waves of the high-frequency oscillations, which half-waves appear at the second transfer coil. The transistor is thus converted to a defined switching condition during the signal duration, which condition can be evaluated as a signal.
A further known possibility for transmitting of d.c. signals lies in the use of a transmitter, which transmits the on and off operations of the d.c. signals and thus permits impulselike signal simulations. However, to overcome signal distortions which occur requires a high amount of circuit components.
Further, it is possible to feed a transmitter for transmitting d.c. signals from a special generator having high frequency oscillations which, corresponding with the d.c. signals, are controlled on the primary side and deliver on the secondary side after rectification again a d.c. signal which corresponds to the supplied d.c. signal. However, in such circuits, due to the preoscillation condition of the high-frequency generator in connecton with the transmitter, the transmitting speeds are held within relatively low limits. To increase the transmitting speed, it is also possible to provide accelerating circuits, which improve the preoscillation condition of a generator, however, also increase the expense of the circuit.
German Auslegeschrift No. 1 244 242 discloses an arrangement for transmitting d.c. signals, which operates with a feed-back generator and transforms impulse combinations into square-wave currents or sinusoidal currents, whereby also a re-rectification after transformation is used. This arrangement operates with a transmitter, in which high-frequency oscillations are produced with the aid of an amplifier and in which a control occurs on the primary side by the d.c. signals, so that on the secondary side after rectification of the oscillations again a d.c. signal is available. The d.c. signal effects thereby on the primary side through different damping a use or nonuse of the oscillations.
The known possibilities for the undergrounded transmission of signals through testing points have the common disadvantages of a limited transmitting speed and only a limited possible analogue reproduction of the signal after the testing point. Further, a relatively high input voltage is required to effect switching operations on semiconductor switching elements or fast signal damping.
The purpose of the invention is to design a circuit arrangement, through which the transmission of analogue or digital signals through testing points is possible with the least possible expense and at a high transmitting speed and effecting a linear transmission of even the smallest input signals, for example, measuring signals.
A circuit arrangement of the above-mentioned type is constructed inventively to attain this purpose such that a resonant circuit which is adjusted to the high-frequency oscillations, and which is galvanically separately coupled to a high-frequency generator, is connected to a modulation circuit which is connected to the input of the testing point and is coupled inductively to a demodulation circuit defining the output of the testing point.
A circuit arrangement of this type operates in such a manner, that a high-frequency generator feeds its high-frequency oscillations into the resonant circuit and modulated by the modulation circuit dependent on the signals which occur at the input of the testing point. The modulated high-frequency is coupled inductively onto the demodulation circuit, which then exactly reproduces the input signals at the output of the testing point by eliminating the high-frequency part. Through this it is possible, depending on the construction or sensitivity of the modulation circuit, to transmit signals of variable amplitude through the testing point and thus to process analogue and/or digital signals. By the subsequent demodulation, it is assured that the course of the output signals follows exactly the course of the input signals because, through the modulation operation, the amplitude of the input signals determines the amplitude of the high-frequency oscillations which are uncoupled from the resonant circuit.
A circuit arrangement according to the invention can be very compactly constructed by utilizing small parts to directly form the testing point, because it is a high-frequency circuit. The inductance of the resonant circuit can be arranged with the associated coupling coils on a common, very small high-frequency coil core.
A further development of the invention is characterized by the modulation circuit being a MOS field effect transistor which is connected in parallel with the resonant circuit preferably through a diode. This further development reliably prevents a transfer of the high-frequency operations which take place in the testing point onto a line which is, for example, connected to the input. A MOS field effect transistor does not have a rectifier effect between its operating circuit and its control circuit, through which also a reaction freedom in relation to a d.c. voltage offset is assured. Further, the connection of the field effect transistor to the resonant circuit can take place preferably through a diode, which assures that the MOS field effect transistor is driven also with a d.c. voltage component and a resulting optimum function. The use of the MOS field effect transistor offers the additional advantage that an optimum amplification is possible already in the condition of a missing pre-existing voltage on the control electrode. Thus it is possible to connect at the input of the testing point signals with a changing polarity, which change in both possible directions to the potential of the control electrode and with this change effect a corresponding modulation of the high-frequency oscillations on the resonant circuit.
The invention can be realized in the afore-described construction wherein the control electrode of the MOS field effect transistor is connected to the input of the testing point and to a limiting circuit. The use of such a limiting circuit is particularly preferable when the testing point is connected to communication lines which can possibly be statically charged. The limiting circuit reliably prevents the MOS field effect transistor from being damaged at its control electrode.